Polyacrylonitrile fibres

ABSTRACT

A METHOD OF IMPROVING THE RESISTANCE TO DISCOLOURATION BY HEAT OF A WET-SPUN CARBOXYLATE ACRYLIC FIBRE, COMPRISING IMPREGNATING THE GEL-STATE FIBRE WITH A SOLUTION OF A SALT OF A METAL OF GROUP II OF THE PERIODIC TABLE. THE PREFERRED SALTS ARE THOSE OF CA, MG, BA, SR AND ZN. THE SALT RETAINED BY THE FIBRE MAY BE FROM HALF TO TWICE THAT STOICHIOMETRICALLY REQUIRED TO REACT WITH THE AVAILABLE CARBOXYLATE GROUPS OF THE FIBRE.

United States Patent 3,681,004 POLYACRYLONITRILE FIBRES Brian J. D. Torrance, Coventry, England, assignor' to Courtaulds Limited, London, England No Drawing. Filed Jan. 12, 1970, Ser. No. 2,423 Claims priority, application Great Britain, Jan. 20, 1969, 3,067/69 Int. Cl. D06p /00 US. Cl. 8-17 5 Claims ABSTRACT OF THE DISCLOSURE This invention is concerned with improving the resistance to discolouration by heat of acrylic fibres containing covalently bound carboxylic acid groups or alkali metal salts thereof. Such fibres are referred to hereinafter as carboxylate acrylic fibres.

carboxylate acrylic fibres are of copolymers containing at least 85 percent by weight of acrylonitrile and at least 0.5 percent by weight of an ethylenically unsaturated carboxylic acid, for example acrylic acid, an a-alkyl acrylic acid such as methacrylic acid and an Ot-SllbStlIlltCd alkyl acrylic acid, such as itaconic acid. Optionally there may be included in the copolymer one or more ethylenically unsaturated monomers, for example styrene, vinyl acetate, or an ester of acrylic or methacrylic acid for example methyl methacrylate or methyl acrylate.

It is known that carboxylate acrylic fibres are prone to discolour on being heated above 130 C., and as several textile finishing treatments now require that the textiles be heated to above this temperature, methods of increasing the resistance to discolouration by heat of the carboxylate acrylic fibres are of interest.

We have found such a method.

According to the present invention, a wet-spun, gelstate carboxylate acrylic fibre is impregnated with a solution of a salt of a metal from Group II of the Periodic Table, and the fibre is dried. v

The preferred salts are those of Ca, Mg, Ba, Sr and Zn.

We understand the process to involve the chelation of the divalent cation of the salt on the carboxylic acid groups of the carboxylate acrylic compolymer, the chelate being less active as a catalyst to the intramolecular degradation of the copolymer by heat, than is the free carboxyl group or alkali metal carboxylate group of the original polymer, so that the salt impregnated copolymer is more resistant to discolouration by heat.

The formation of the chelate by a double decomposition reaction between the copolymer and the salt leads to the liberation of the anion of the salt as the free acid or alkali metal salt and this by-product is not permanent in the fibre; it is largely washed from the fibre during the impregnation and subsequent wet-processing. However there is evidence that the resistance to discolouration by heat is greater when the impregnant salt is that of a strong acid (having a dissociation constant (K) of at least 1 10- rather than a carboxylic acid. Thus the salt is preferably a sulphonate such as p-toluene sulphonate or benzene sulphonateor an inorganic salt, for example sulphate, chloride or nitrate; the carboxylate salts,

Patented Aug. 1, 1972 for example acetate and benzoate, improve the resistance of the carboxylate acrylic fibre to a lesser extent.

The resistance to heat-discolouration is significantly increased even when only a proportion of the carboxylate groups in the carboxylate acrylic copolymer are incorporated in chelate groups. However the maximum resist ance is developed when all the available carboxylate groups (free acid groups and alkali metal salt groups) are deactivated.

It is preferred that the salt retained by the fibre should be from one half to twice that stoichiometrically required to form the chelate with the available carboxylate groups. Larger proportions are acceptable but do not significantly affect the outcome, except perhaps to accelerate the formation of the chelate.

The deactivation of the carboxylate groups as degradation catalysts following the impregnation of the gel-state fibre with the salt, also reactivates the carboxylate groups as dye sites for basic dyes. In a preferred modification of this process, therefore, the gel state fibre is first dyed and then impregnated with the salt of the Group II metal. Carboxylate groups already occupied by dye molecules are not significantly susceptible to reaction with the salt so that these groups are not available as they were before dyeing. The reaction of the dyed fibre with the salt therefore involves only the supernumerary carboxylate groups, and the stoichiometric proportions of the salt should be calculated on this basis.

It is also possible to partially deactivate the carboxylate groups of the fibre by reaction with the salt and to dye the fibre at some later stage, not necessarily the gel state fibre, but after it has been dried-but ideally this process requires a knowledge of the intended dyeing and the required content of dyeable carboxylate groups-facts not easily ascertained at the fibre-making stage.

Attendant benefits of the process are that certain dyes which change shade on carboxylate acrylic fibres subjected to heat, are no longer so unstable on the salt impregnated fibres. The salt impregnated fibres are also resistant to dyeing to a degree depending on the proportion of carboxylate groups incorporated in the chelate groups and this property may be employed to obtain parti-coloured yarns and fabrics by piece dyeing, when the dye resistant fibres are blended with dyeable fibres.

The effectiveness of the treatment was demonstrated in a series of tests in which multifilament yarns treated in accordance with this invention, and comparable untreated yarns, were mounted together on a rotor in an oven heated to a selected temperature in the range of to C. Samples of both yarns were removed at five minute intervals from the oven, the first at five minutes and the last at 30 minutes. This procedure was repeated at other oven temperatures within the stated range. From the spectrum of results is was possible to select samples of untreated yarn which had discoloured to the same extent within similar dwell times in the oven. In all such cases the temperature required to discolour the treated sample was higher than that which discoloured the untreated sample and this difference in temperature is exemplified in the following examples which identify the impregnating salt.

EXAMPLE 1 20 grams of never-dried carboxylate acrylic fibre con taining 2 percent by weight of acrylic acid on the dry fibre weight was immersed for 30 seconds in a 2 percent by weight zinc sulphate solution at 20 C. The fibres, after washing in demineralised water and drying in air at a temperature of 80 C., were 0.5 gm. heavier than another 20 gm. sample of untreated fibers which had been dried in the same way. The treated and untreated fibres were subjected to the heat stability test referred to above. The

treated fibres required to be heated at a temperature twenty centigrade degrees above that which discoloured the untreated fibre to the same extent in the same time. EMMPLE 2 The procedure of Example 1 was followed, substituting other salt solutions for the zinc sulphate solution. The solutions are identified in the following table, together with the difference in temperatures at which the treated and untreated samples discoloured to the same extent and in the same time.

What is claimed is:

1. A method of improving the resistance to discolouration by heat of a wet-spun acrylic fibre containing carboxylic acid groups comprising impregnating the gel-state fibre with a solution of an uncolored, non-reducing salt of a metal of Group II of the Periodic Table.

2. A method as claimed in claim 1 in which the salt has a cation chosen from the group C-a++, Mg++, Ba++, Sr++ and Zn++ and :an anion derived from an acid having a dissociation constant (K) of at least 1 10- 3. A method as claimed in claim 1 comprising dyeing the gel-state fibre before impregnating it with the salt solution.

4. A method as claimed in claim 1 in which the salt retained by the fibre is from half to twice that stoichiometrically required to react with the available carboxylic acid groups of the fibre.

5. An acrylic fibre containing carboxylic acid groups and Group II metal cations in a proportion equal to onehalf the stoichiometric equivalent of said carboxylic acid groups.

References Cited UNITED STATES PATENTS 3,297,471 1/1967 Traumann 117-62.1 3,410,647 11/1968 Hirshfield s-97 3,091,552 5/1963 Furness 117 13s.s

DONALD LEVY, Primary Examiner U.S. Cl. X.R. 

